feat(gui2): Integrate core event system

Integrates the ai_client.events emitter into the gui_2.py App class. Adds a new test file to verify that the App subscribes to API lifecycle events upon initialization. This is the first step in aligning gui_2.py with the project's event-driven architecture.

gui_2.py

@@ -14,6 +14,7 @@ import session_logger
 import project_manager
 import theme_2 as theme
 import tomllib
+import events
 
 from imgui_bundle import imgui, hello_imgui, immapp
 
@@ -187,6 +188,15 @@ class App:
         ai_client.comms_log_callback = self._on_comms_entry
         ai_client.tool_log_callback = self._on_tool_log
 
+        # Subscribe to API lifecycle events
+        ai_client.events.on("request_start", self._on_api_event)
+        ai_client.events.on("response_received", self._on_api_event)
+        ai_client.events.on("tool_execution", self._on_api_event)
+
+    def _on_api_event(self, *args, **kwargs):
+        """Callback for ai_client events. For now, a placeholder."""
+        pass
+
     # ---------------------------------------------------------------- project loading
 
     def _load_active_project(self):